Knee extensor muscle oxygen consumption in relation to muscle activation

Recently, fatigability and muscle oxygen consumption (mVO₂) during sustained isometric contractions were found to be less at shorter (30° knee angle; 0° = full extension) compared to longer knee extensor muscle lengths (90°) and, at low torques, less in the rectus femoris (RF) muscle than in the vastus lateralis and medialis. In the present study we hypothesized that these findings could be accounted for by a knee angle- and a muscle-dependent activation respectively. On two experimental days rectified surface EMG (rsEMG) was obtained as a measure of muscle activation in nine healthy young males. In addition, on day 1 maximal torque capacity (MTC) was carefully determined using superimposed nerve stimulation on brief high intensity contractions (> 70%MVC) at 30, 60 and 90° knee angles. On day 2, subjects performed longer lasting isometric contractions (10–70%MTC) while mVO₂ was measured using near-infrared spectroscopy (NIRS). At 30°, maximal mVO₂ was reached significantly later (11.0 s ± 6.5 s) and was 57.9 ± 8.3% less (average ± SD, across intensities and muscles) than mVO₂ at 60 and 90° (p < 0.05). However, rsEMG was on average only 18.0 ± 11.8% (p = 0.062) less at the start of the contraction at 30°. At 10%MTC at all knee angles, maximal mVO₂ of the RF occurred significantly later (28.8 ± 36.0 s) and showed a significantly smaller increase in rsEMG compared to both vasti. In conclusion, it is unlikely that the tendency for less intense muscle activation could fully account for the ∼60% lower oxygen consumption at 30°, but the later increase in RFmVO₂ seemed to be caused by a less strong activation of the RF.